Colloidal nanoparticles are nanometer-sized solid particles which are of interest for advanced materials applications. Generally, colloidal nanoparticles include colloidal nanocrystals as well as amorphous particles, and can comprise either inorganic or organic solids. For simplicity, inorganic nanocrystals are discussed as the representatives of nanometer-sized solids. The term “nanometer-sized” is typically used to refer to particles with an approximate size range between about 1 nm to about 1000 nm in diameter (one nanometer is 10−9 meter). More typically, “nanometer-sized” refers to an approximate size range between about 1 nm to about 100 nm in diameter.
Usually, nanocrystals are thermodynamically metastable and typically need kinetic stabilization with a monolayer of organic ligands, typically referred to as capping groups, surfactants, stabilizers, and the like. These organic ligands function by having at least one binding site with which to bind the surface atoms of the nanocrystals, while the remaining portion of the ligand molecules provides steric isolation between nanocrystals.
Organic dendrons are regularly branched organic molecules starting from a focal point or molecular branching point, which can be used as ligands for nanocrystals. For simplicity, the term dendron is used to refer to any organic ligand that is substantially branched. With organic dendrons, the molecular focal point typically constitutes the site at which the ligand binding occurs while the periphery constitutes the organic groups that provide steric protection. The number of branching points from the focal point to the terminal groups of the dendron is used to characterize these ligands and is referred to as the number of generations of the dendrons. For instance, generation 3 (G3) dendrons refers to dendrons with three branching points from the focal point to the terminal groups of the dendrons.
Colloidal nanocrystals have generated great fundamental interest in recent years and continue to exhibit tremendous promise for developing advanced materials for a variety of important technical applications. The size-dependent emission is probably the most attractive property of semiconductor nanocrystals. For example, differently sized CdSe nanocrystals can be prepared that emit from blue to red with very pure color. These nanocrystal-based emitters can be used for many purposes, such as light-emitting diodes, lasers, biomedical tags, and the like. Further, magnetic nanocrystals can be used as enhancing agents for magnetic resonance imaging (MRI) in the medical diagnostics industry.
Current research and development activities in colloidal nanocrystals remain hindered by the paucity of reliable processing chemistry. Recent developments in the synthesis of semiconductor, metal, and oxide nanocrystal systems, have placed the need for better processing chemistry in an even more urgent position. The processibility of colloidal nanocrystals is related to the stability of the nanocrystal-ligand complex, which it turn is related to the nature and structure of the surface ligand monolayer, the interface between the inorganic core and the organic ligand monolayer, and the structure of the surface of the inorganic nanocrystals themselves. Yet, knowledge regarding the structure of inorganic nanocrystal surface, the nature of the nanocrystal-ligand interface, and the dynamics and structure of the ligands within the monolayer is limited.
An example of the importance in obtaining stable nanocrystal-ligand complexes is seen in the lifetime of the light emitting diodes (LEDs) based on semiconductor nanocrystals. Presently, this attainable lifetime is relatively short, which is likely the result of the thermal dissociation of organic ligands from the nanocrystal surface during operation of the LED device. Further, the troublesome conjugation chemistry related to promising biological labeling applications using semiconductor nanocrystals has been also found to be associated with the detachment of the organic ligands from the nanocrystal surface. Additionally, the long-pursued enhancement effect of magnetic nanocrystals for magnetic resonance imaging is still in its infancy because of the instability of the ligands on the surface of the nanocrystals.
Two types of stability/instability issues related to nanocrystal/ligand complexes have been identified. Type I instability arises when the organic ligands dissociate from the inorganic core, and the nanocrystal/ligand complex is thus destroyed. In solution, this type instability results in uncontrollable chemical properties of the outer surface of the ligand monolayer and the detachment of the desired chemical/biochemical functions from the inorganic core. In many cases, this Type I instability also induces precipitation of the nanocrystals. In both solid state and in solution, dissociation of the organic ligands and the inorganic core often causes undesired variations of the properties of nanocrystals, such as decrease of either photoluminescence or electroluminescence brightness. Type II instability is associated with the inorganic core of the nanoparticle or nanocrystal being subject to oxidation, etching, and even complete dissolution. Thus, Type II instability normally defeats the function of the original nanocrystal/ligand complex.
Therefore, what is needed are new processing methods to impart and maintain greater stability to the nanocrystal/ligand complex. Both Type I instability resulting from dissociation of organic ligands from the inorganic core, and Type II instability arising from the chemical oxidation, etching, or dissolution of the inorganic core following ligand dissociation should be addressed. Nanocrystal stability is closely associated with the nature and structure of the surface ligand monolayer, the interface between the inorganic core and the organic ligand monolayer, and the structure of the surface of the inorganic nanocrystals themselves, therefore all these aspects of nanocrystal stability should be addressed. If possible, these new processing methods would be applicable to enhance the thermal stability, the redox stability, and the photolytic stability of the nanocrystal/ligand complex.